Melon (Cucumis melo L.) is a commercial crop grown worldwide. Cucumis melo is a member of the family Cucurbitaceae. The Cucurbitaceae comprises about 90 genera and 700 to 760 species, mostly of the tropics. The family includes pumpkins, squashes, gourds, watermelon, loofah and several weeds. Cucumis melo L. includes a very wide variety of cultivars producing fruits of different shape, external appearance and flesh color. Commercial melons generally produce sweet fruits known for example as Charentais, cantaloupe, honeydew, amarello, Piel de sapo, Kirkagak, Hamy, Ananas, Galia, Oriental that are usually consumed as dessert fruits. Cucumis melo L. also includes non-sweet, commercial cultivars consumed in the Middle to Far East in salad, cooking or pickling, as for example Alficoz, Faqqous, Chito, Conomon (Pitrat et al (2000) Eucarpia meeting Proceedings: 29-36). The taste and aroma of melon fruits is determined by a number of factors, including sugars, aroma volatiles, free amino acids, organic acids, pH and soluble minerals (Wang et al. (1996) J. Agric. Food Chem. 44: 210-216). Among the four primary tastes (sweet, sour, bitter, salty), sweetness is considered to be a very important component of good tasting melon fruits. In commercial melon fruits the sweet taste mostly results from high levels of sucrose (Burger et al. (2002) J. Amer. Soc. Hort. Sci. 127(6): 938-943). Sucrose is accumulated at the end of fruit development, during ripening process (Shaffer et al (1987) Phytochemistry 26: 1883-1887). Melon fruits also initially accumulate hexoses, mainly fructose and glucose, which are the dominant reducing sugars (Stepanski et al (1999) Genetic Resources and Crop Evolution 46: 53-62). An important taste component in melon fruits is sweetness, which is mainly the result of sugars accumulation. Sweetness correlates not only with the total sugars content, but also with the type of sugars. For example, 1 gram of glucose is the sweet equivalent of 0.7 gram of sucrose; 1 gram of fructose is the sweet equivalent of 1.7 gram of sucrose; 1 gram of inverted sugar, i.e. glucose plus fructose generated from 1 gram of sucrose, is the sweet equivalent of 1.3 gram of sucrose (J. A. BABOR et J. IBARZ (1935) Quimica General Moderna).
The flesh of sweet melon fruits has a pH usually above 6.0, but melon accessions are also known to have a much lower pH, as low as below 5.0. This low pH is widespread over many different melon types as for example Faqqous, Chito, Conomon, Momordica, Agrestis (Stepanski et al). In most of these cases, these melon types combine low pH and low sugar content, e.g. sucrose (Stepanski et al). These fruits are generally not edible in fresh consumption without dressing or cooking and, in some cases, they are even bitter. In most of these melon accessions with low pH, the mesocarp, which is the edible part of the fruit, represents a minor part of the total fruit, while the seed cavity and placenta represent a major part of the total fruit fresh weight. This is in contrast to sweet dessert melons, where the mesocarp represents a major part of the fruit. Also, in many cases, the fruit size or weight of the melon having low pH is below commercially acceptable ranges.
The fruit flesh of some melons has a sour taste (Kubicki (1962) Genetica Polonica 3:265-274). The cause for the sour taste remains unclear, but it has been linked with low pH in the fruit flesh (U.S. Pat. No. 5,476,998 and Danin-Poleg et al. (2002) Euphytica 125: 373-384). Single genes for sour taste (So) and pH have also been reported, although their genetic association is not clear (Danin-Poleg et al.).
Attempts have been made to produce melon fruits combining sour and sweet tastes. For example, Najd melons based on Arabic wild varieties have been reported (Ibrahim and Al-Zeir (1992) HortScience 27: 276-277). U.S. Pat. No. 5,476,998 also described melons with a sour taste, with a mean pH value of 4.8 and a total soluble content of about 11. The melons in U.S. Pat. No. 5,476,998 are derived from MR-1, also known as PI124111 (Thomas, Eucarpia '92 pp 142), which is a C. melo Var. Momordica (Roxburg). Fruits of this melon showed low pH (Danin-Poleg et al.). This melon variety also involves poor fruit traits such as very mealy, non-aromatic and non-sweet flesh, very climacteric behavior and thin skin bursting at ripening. These undesired traits are difficult to remove by breeding and it is therefore expected that the development of commercial products from such a variety would be lengthy and difficult. Accordingly, there is an unmet need for melons producing fruits with alternative or improved tastes. In particular, there is an unmet need for melon fruits having new combinations of organoleptic characteristics and aroma.